1. Field of the Invention
The present invention relates to a frequency hopping spread spectrum (FHSS) scheme for radio frequency identification (RFID) devices, and more particularly to a system and method for improving transmission rates in an RFID device by implementing forced frequency “hops.”
2. Description of Related Art
Radio Frequency Identification (RFID) base stations, which operate in conjunction with RFID transponders (tags), are used in a variety of applications (e.g., inventory-control, security, etc.). Typically, an item including an RFID tag (e.g., a container with an RFID tag inside) is brought into a “read zone” established by the base station. The base station transmits an interrogating RF signal that is modulated, in part, by the receiving tag. That is, in reading the tag, the base station generates and transmits a continuous wave electromagnetic disturbance at a particular carrier frequency. This disturbance is then modulated by the receiving tag in order to impart information (e.g., information stored within the tag) into the signal. The modulated RF signal is then reflected back to the base station where the imparted information is extracted.
When interrogating an RFID tag, however, the amount of time that can be spent on a particular carrier frequency is regulated by the Federal Communications Commission (FCC). The FCC, at least with respect to the Instrumentation, Scientific and Medical (ISM) bands, only allows transmitters operating under the Frequency Hopping Spread Spectrum (FHSS) methodology to continuously transmit over a particular carrier frequency for up to four hundred milliseconds. After that, the transmitter is required to transmit over a different carrier frequency (i.e., “hop” to a new carrier frequency). For example, a transmitter could transmit an RF signal for four hundred milliseconds over a first frequency, “hop” to a second frequency, transmit an RF signal for four hundred milliseconds over the second frequency, “hop” to a third frequency, etc.
One of the drawbacks associated with traditional FHSS systems is that time is wasted when the transmitter is not transmitting (i.e., when the transmitter is “dwelling”). Dwelling results from two common FHSS characteristics—(i) frequency synchronization and (ii) transmission/allocation time variance. First, FHSS systems are traditionally designed to operate using synchronized frequencies. In other words, in most FHSS systems, the frequency at which the transmitter is transmitting needs to be same as (or synchronized with) the frequency at which the receiver is receiving. This is typically achieved by using a common algorithm (i.e., an algorithm known to both the transmitter and the receiver). For example, a common algorithm may dictate that the available bandwidth (i.e., the frequency spectrum) is to be divided into seventy-five channels (i.e., seventy-five frequencies), and that the first communication cycle is to be transmitted over the first channel for four hundred milliseconds, the second communication cycle is to be transmitted over the second channel for four hundred milliseconds, etc.
By following the common algorithm, the transmitter and the receiver are able to “hop” from channel to channel in unison. If the transmitter were to “hop” early (e.g., “hop” to the second channel before the elapse of four hundred milliseconds), the system would become unsynchronized. In other words, the transmitter would be transmitting data over the second channel while the receiver (in accordance with the algorithm) would be attempting to receive data over the first channel. Therefore, it is imperative that the transmitter follow the common algorithm and remain at each channel for the prescribed amount of time.
This raises the next issue—i.e., transmission/allocation time variance, or whether there is a variance between the time it takes to transmit data and the time allotted (or prescribed) by the common algorithm. For example, if the allotted time is four hundred milliseconds and it takes one hundred and fifty milliseconds to transmit one item of data, then two items of data (i.e., three hundred milliseconds worth) is all that can be transmitted over a single channel. This is because it would take four hundred and fifty miliseconds to transmit three items of data, which would violate the time allotted (i.e., pursuant to FCC regulations). This results in a dwelling period of one hundred milliseconds per channel (i.e., the allotted time minus the transmitting time equals the dwelling period).
While dwelling periods are often associated with systems having the above-mentioned characteristics, they are not limited to such systems. For example, an FHSS system that does not require frequency synchronization may still experience dwelling periods. This is because non-synchronized FHSS systems typically operate under the assumption that maximum transmission rates are achieved by minimizing the “hop” rate. In other words, a maximum transmission rate will be achieved by spending more time transmitting data and less time “hopping” from channel to channel.
Traditional RFID systems operating under the FHSS methodology do not require frequency synchronization. This is because RFID tags can be designed to receive, without “hopping,” signals transmitted over various carrier frequencies. This leaves the RFID base station free to “hop” unilaterally (i.e., without using a common algorithm). Nonetheless, because such systems typically operate under the assumption that maximum transmission rates are synonymous with minimum “hop” rates, algorithms are employed to “hop” only after the expiration of the time allotted by the FCC (i.e., four hundred milliseconds). The drawback with such a system (i.e., fixing the “hop” period) is that a dwelling period is created if the transmission time is not equal to the time allotted by the FCC, as previously explained.
Thus, it would be advantageous to provide an FHSS scheme that forced RFID base stations to “hop” instead of “dwell” when timing constraints prohibit the transmission of additional data.